1. Field of the Invention
The present invention relates to a heat-transfer-medium heating apparatus for heating a heat transfer medium using a positive temperature coefficient (PTC) heater, and to a vehicular air-conditioning apparatus using the same.
This application is based on Japanese Patent Application No. 2006-234151, the content of which is incorporated herein by reference.
2. Description of Related Art
One known heat-transfer-medium heating apparatus for heating a heat transfer medium in the related art is an apparatus using a PTC heater, in which a PTC thermistor device (PTC element) is used as a heat-generating element.
In a PTC heater, which exhibits a PTC thermistor characteristic, the resistance increases as the temperature increases, and the temperature rise is moderated as the consumed current is controlled by this increase in resistance. Thereafter, the temperature of and current consumed by the heat-generating part reach a saturation region where they are stabilized. The PTC heater thus exhibits a self-controlled temperature property.
As described above, the PTC heater has a property whereby the current consumed decreases as the temperature of the heater rises, and then when it reaches a constant-temperature saturation region, the current consumed stabilizes at a low value. Making use of this property can afford an advantage in that it is possible to reduce the electrical power consumption, as well as preventing an abnormal rise in the temperature of the heat-generating part.
Because they exhibit such features, PTC heaters are used in numerous technical fields. In the field of air conditioning, for example, vehicular air-conditioning apparatuses, they have also been proposed for use in a heating apparatus for heating a heat transfer medium (in this case, engine coolant) supplied to a radiator for heating air (for example, see Japanese Unexamined Patent Application, Publication No. 2003-104041).
In the technology described in Japanese Unexamined Patent Application, Publication No. 2003-104041, an indentation for placing the PTC heater is provided in a compartment wall dividing the flow path of a fluid, and the PTC heater is disposed in this indentation to heat the coolant flowing in the flow path via the compartment wall.
In this case, although it is possible to increase the heat-conducting surface area in the fluid flow path, there is inevitably some difficulty in assembling the structure by inserting the PTC element, serving as the heat-generating element, in the indentation and placing it in close contact with the surface of the compartment wall. Therefore, this approach has some problems that must be overcome, such as heat-conduction to the fluid flow path, the ease of assembly, and so forth.
In addition, when the heating apparatus described above is employed in the air-conditioning apparatus of an electric car, a high voltage, for example, 300 V, is applied to the PTC heater. Therefore, one critical issue is ensuring sufficient electrical insulation between the PTC heater and the fluid flow path. However, this issue is not described at all in Japan Unexamined Patent Application, Publication No. 2003-104041.